This large museum has a surprising amount of English in its narration of the history of the DMZ, as well as exhibits such as US POW letters and extensive photos.
It's inside the Tongil Security Park, on the left side of the road as you approach the Goseong Unification Observatory.
From downtown Sokcho or the bus stop right outside the intercity bus terminal, catch bus 1 or 1-1 (₩5020, 1½ hours, 44km, every 15 minutes) headed north, but ask if they go to the DMZ as not all do. Get off at Machajin (마차진; a round-trip taxi from Sokcho might cost in the region of ₩70,000) and walk about 10 minutes up to the Tongil Security Park (통일안보공원). Here you present identification and purchase your admission ticket. If you don’t have your own vehicle, the staff might be able to help you hitch a ride, but don’t count on it. It’s 10km to the observatory; pedestrians, bicycles and motorbikes are not allowed.